In a fast reactor that uses metal sodium as coolant, an electromagnetic pump is employed to circulate the coolant.
Making use of the fact that liquid metal sodium is a good conductor of electricity; the electromagnetic pump is designed to transport the metal sodium coolant based on the principle that the electromagnetic pump receives a force in proportion to a magnetic field strength thereof in a direction perpendicular thereto when the electromagnetic pump through which a current flows is placed in a magnetic field.
The electromagnetic pump has excellent characteristics compared with a conventional mechanical pump. In terms of functionality, flow rate of coolant can be easily and linearly adjusted. In terms of structure, it is possible to keep the coolant in a completely sealed state. Because the electromagnetic pump is small in size, the electromagnetic pump may be combined with another apparatus, such as steam generator, for the purpose of streamlining. The electromagnetic pump does not have moving parts, and is therefore easy to maintain and repair. And electromagnetic pump is possible obtain a high discharge pressure.
When abnormality, such as plant trip or loss of offsite power, occurs in a running fast reactor, a function of ensuring flow rate during a predetermined period of time after shutdown of the reactor, i.e. flow coast down characteristic function, is required to mitigate a transitional coolant temperature difference at an inlet and outlet of a reactor core that is generated as a control rod is quickly inserted into the core of the reactor.
As described above, when the control rod is quickly inserted into the core, core outlet temperature would increase if the flow rate through the core decreases more rapidly relative to the degree of attenuation of heat removal from the core. In order to mitigate the above, the attenuation speed of the flow rate needs to be consistent with the amount of heat generation. Such a characteristic is known as flow coast down characteristic.
The flow coast down characteristic is important not only in terms of safety in avoiding an increase in the core outlet temperature, but also in terms of ensuring soundness in mitigating structural-material temperature change, i.e. mitigating plant thermal transient in a system, particularly in a sodium-cooled reactor where a heat transfer rate between coolant sodium and structural material is high.
As for the function of ensuring a flow rate during a predetermined period of time after shutdown of the reactor, flow coast down characteristic function, some kind energy storage needs to be separately prepared to ensure an attenuating output characteristic of an electromagnetic pump because the electromagnetic pump has no rotating parts and no mechanical inertia.
Moreover, the electromagnetic pump is an extreme inductive load, and power factor thereof therefore is about 0.5, which is smaller than power factor of a conventional mechanical sodium pump.
In the case that the electromagnetic pump is used as a pump requiring large capacity such as a coolant circulation pump of a fast reactor, driving power supply capacity thereof would become massive unless a power factor correction means is provided to compensate reactive power. As a result, the problem is that it is difficult to lay out power equipment, and design work could be complicated, so the use of the pump would become disadvantageous from an economical point of view.
As mentioned above, as an energy storage means that can ensure the flow coast down characteristic to mitigate plant thermal transient and improve a low power factor of the electromagnetic pump at the same time, there is a method of using a superconduction energy storage apparatus, and supplying to the electromagnetic pump via a power converter control apparatus from energy stored in a superconductive electromagnet as disclosed in Patent Document 1 for example.
A synchronous machine connected parallel to the electromagnetic pump may be operated as a phase modifier during normal operation of a plant. When abnormality such as cutting-off of a power supply circuit of the electromagnetic pump has occurred, as disclosed in Patent Document 2 for example, there is a method of regenerating an excitation current from rotation energy, and supplying the excitation current to the electromagnetic pump.